The present invention relates to hydrostatic power steering devices such as fluid controllers and torque generators, and more particularly, to such devices which are used in hydraulic circuits with other fluid pressure actuated devices.
Typical hydrostatic power steering devices include a housing defining inlet and return ports, some sort of fluid pressure displacement mechanism such as a gerotor gear set, and valving to control the flow of fluid from the inlet port through the gerotor to the return port, in response to rotation of a steering wheel. In the typical, prior art device, leakage fluid within the device (e.g., fluid leaking from the gerotor) collects in the interior or case drain region of the device, and then is communicated through the valving to the return port of the device from where it returns to the system reservoir. Such devices also typically include bearings and seals disposed between the housing and valve means, with the bearings and seals being subjected to the fluid pressure in the case drain.
In many vehicle hydraulic circuits which include hydrostatic power steering devices, it is desirable to include additional fluid pressure actuated devices (such as a garden implement or winch) in the same circuit so that only a single pump is required, but disposed downstream of the steering device, so that the steering device always has priority. It is most desirable to connect such a downstream, auxiliary device to the return port of the steering device, which greatly simplifies the circuit plumbing, as well as the steering device. Unfortunately, if the auxiliary device operates on a pressure differential such as 1,000 psi or more, the result is, with prior art steering devices, that there will be 1,000 psi back pressure present in the return port of the steering device, which is acceptable, but also there will be 1,000 psi present in the case drain, which is not acceptable, primarily because the bearings and seals would be exposed to such a pressure.